This invention relates to a system for detecting the concentration of oxygen in gases by using an oxygen sensor having a solid electrolyte cell which acts as an oxygen partial pressure detector and another solid electrolyte cell which acts as an oxygen ion pump by application of a current thereto. This system is suitable for use in detecting the air/fuel ratio in an internal combustion engine by detecting the concentration of oxygen in the exhaust gas.
For use in various combustion gases, oxygen sensors using an oxygen ion conductive solid electrolyte such as zirconia are well known. In the current automotive internal combustion engines, it is popular to perform feedback control of air/fuel ratio by using an oxygen sensor to estimate an actual air/fuel ratio by detecting the concentration or partial pressure of oxygen in the exhaust gas.
As is well known, an oxygen sensor for use in exhaust gases can be constructed by forming an electrode layer on an outer surface of a tubular body of zirconia or an alternative ceramic solid electrolyte and another electrode layer on the inner surface. The tubular body is closed at one end, and the outer side of this oxygen sensor is exposed to the exhaust gas while the inner side is exposed to a reference gas such as the atmospheric air. When the nominal air/fuel ratio in the engine is nearly stoichiometric, this sensor can be used in the manner of an oxygen concentration cell that generates an electromotive force between the two electrodes according to a difference between the partial pressures of oxygen on the outer and inner sides of the zirconia tube without need of applying any external voltage or current to the sensor. However, when the nominal air/fuel ratio is substantially higher than the stoichiometric ratio as in the cases of so-called lean-burn engines this oxygen sensor has to be used in a different manner. For example, it is known to apply a constant voltage across the outer and inner electrodes of the sensor to measure a current that is produced by the constant voltage and flows through the solid electrolyte. The current varies proportionally to the concentration of oxygen in the exhaust gas, and, insofar as the air/fuel ratio in the engine is not lower than the stoichiometric ratio, the oxygen concentration in the exhaust gas is nearly proportional to the air/fuel ratio.
In practice, however, there is a problem that the oxygen-sensitive characteristic of the sensor gradually changes during long exposure of the sensor to high temperature exhaust gases due to deposition of solid particles contained in the exhaust gases and/or repeated thermal shocks. Also it is a problem that industrially produced sensors of the same design are liable to be somewhat different from one another in the oxygen-sensitive characteristic. Therefore, some measures should be taken for avoidance of an increase in errors in the feedback control of air/fuel ratio by reason of these problems.
Japanese patent application primary publication No. 58-57050 (1983) relates to feedback control of air/fuel ratio in an automotive engine operated with a fuel-lean mixture and proposes to calibrate the oxygen-sensitive characteristic of the above described oxygen sensor, which is operated with application of a constant voltage thereto, by checking the level of the aforementioned current while the exhaust gas flowing along the outer side of the sensor is equivalent to the atmospheric air. According to the proposal, cutoff of fuel feed is detected in order to make a judgement that an atmospheric condition is realized in the exhaust system at the location of the oxygen sensor after the lapse of a predetermined length of time from the moment of the cutoff of fuel feed. At the time of the calibration there is no change in the magnitude of the constant voltage applied to the oxygen sensor.
This calibration method is good in accuracy since the concentration of oxygen in the air is constant. However, repeated calibration by this method will adversely affect the durability of the oxygen sensor for the following reason. In a lean-burn engine the nominal air/fuel ratio is usually in the range from about 16 to about 24 by weight, so that the concentration of oxygen in the exhaust gas is in the range from about 1% to about 8%. At the time of calibration the exhaust gas under the atmospheric condition contains about 20% of oxygen. Since the constant voltage applied to the oxygen sensor is invariable the current flowing through the oxygen sensor under calibration becomes 2 to 4 times as large as the current flowing during normal operation for detection of air/fuel ratio. Therefore, deteriorating aging of the solid electrolyte and/or electrodes of the oxygen sensor is significantly accelerated.